1. Field of the Invention
The present invention relates to a fixing apparatus used for image forming apparatuses utilizing an electrophotographic process such as copying machines, facsimile machines, and printers.
2. Description of the Related Art
In copying machines and the like utilizing the electrophotographic process, it is necessary to fix a nonfixed toner image formed on a recording sheet to render the image into a permanent image. Generally, a heat fixing method involving the step of fixing the toner by heating to deposit the fixed toner on the recording sheet is extensively used.
As the heat fixing apparatus, well known is a heat roll type fixing apparatus including: a heater inside a cylindrical core member; a heat roll having a heat-resistant resin coating layer on the outer circumferential surface thereof; and a pressure roll being disposed in pressure contact with the heat roll and having a heat-resistant elastic layer formed on the outer circumferential surface of a cylindrical core member thereof. The heat fixing apparatus is designed to deposit a nonfixed toner image by causing a recording sheet having such nonfixed toner image thereon to pass through the space between these rolls.
Such a heat roll type fixing apparatus employs the pressure roll as described above, and an expensive heat-resistant elastic material such as silicone rubber or fluororesin rubber must be used to form the heat-resistant elastic layer on the roll surface in order to manufacture the pressure roll, thereby having the problem of elevated cost of manufacture of the fixing apparatus. In addition, to insure a desirable fixing condition, the width of contact between the heat roll and the pressure roll (i.e., the width of the nipping section) have to be within the range of about 4 to 10 mm, which required, e.g., that the roll diameter is set to rather large values or that a loading mechanism for applying a high load is disposed. This has made it difficult to downsize both the heat roll and the pressure roll, which in turn makes the apparatus large and complicated in structure as a whole.
To overcome the above problems, for example, a fixing apparatus has been suggested, to which applied is a heat-resistant elastic member having a semi-circular shape in section is fixedly disposed in place of the aforementioned pressure roll so that the nipping section can be formed so as to be in pressure contact with the heat roll (Unexamined Japanese Utility Model Publication No. Sho. 61-156675). Since the pressure roll is not necessary, this conventional fixing apparatus has been successful in implementing the cost reduction as well as downsizing of the apparatus.
However, even with this fixing apparatus, various improvements are called for to meet the increasing needs in recent years for a further downsizing, cost reduction, higher performance of the apparatus and the like.
Moreover, how the recording sheet having a nonfixed toner image can reliably and smoothly be threaded in and passed through the nip section formed between the heat roll and the heat-resistant elastic member by pressure contact is another important subject to be overcome with such fixing apparatus. It should be noted that if the conventional supply unit for supplying a release agent to the heat roll and the like is arranged to achieve satisfactory threading and the like of the recording sheet, not only the structure of the fixing apparatus is as much increased in size, but also the cost of manufacture is as much elevated as a whole as the supply unit.